Abstract: In view of the controversial viewpoint of existing research on the effect of Internet use on physical exercise of the elderly ("positive promotion theory" and "negative weakening theory"), this study, based on the China Family Panel Studies (CFPS 2020), divides the physical exercise behavior of the elderly into three dimensions: exercise participation, exercise frequency and exercise intensity to explore different effects of Internet use on their physical exercise as well as its mechanism. It is found that Internet use has a positive and negative dual effect on physical exercise of the elderly: Internet use is conducive to improving the exercise participation and intensity of the elderly, but it will also weaken their exercise frequency for those with exercise habit. The mechanism analysis shows that the use of the Internet promotes the participation of the elderly in exercise by improving social capital, but it will also occupy their leisure time and lead to a reduction in the frequency of exercise. The heterogeneity analysis shows that the Internet has empowered the elderly of different classes in terms of exercise participation. However, in terms of exercise frequency and intensity, there is a more significant reinforcement effect on the elderly with high socio-economic status.